1. Field of the Invention
The present invention relates to an actuator of an antiskid system for a vehicle and, more particularly, the invention pertains to an actuator of an antiskid system for a vehicle having a cut-off valve for selectively dividing a hydraulic pressure circuit connecting together a brake master cylinder and a wheel brake cylinder of the vehicle into a brake master cylinder-side hydraulic pressure circuit portion and a wheel brake cylinder-side hydraulic pressure circuit portion; an accumulator for accumulating a power hydraulic pressure; a piston moved in one direction (referred to as a "first direction", hereinafter) by the hydraulic pressure in the wheel brake cylinder-side hydraulic pressure circuit portion as well as moved in the other direction (referred to as a "second direction", hereinafter), opposite to the first direction, by the power hydraulic pressure so as to close and open the cut-off valve as well as to increase and restore the volume of the wheel brake cylinder-side hydraulic pressure circuit portion by the movement thereof in the first and second directions; and a changeover valve interposed in a power hydraulic pressure circuit connecting together the accumulator and a fluid chamber which is formed on one of the sides of the piston and to which the power hydraulic pressure is applied, the changeover valve being adapted to remove the power hydraulic pressure in the fluid chamber when a wheel of the vehicle is brought into a state close to a locked state and to apply the power hydraulic pressure to the fluid chamber when the wheel is in a state other than one close to a locked state.
2. Description of the Prior Art
In the above-described actuator, when the wheel is brought into a state close to a locked state, the power hydraulic pressure in the fluid chamber is removed. However, when the wheel is in a state other than one close to a locked state, the power hydraulic pressure is constantly applied to the fluid chamber. Although the period of time when a brake is in an operative state is extremely short as compared with the period of time for other operations of the vehicle, the power hydraulic pressure is normally applied to the fluid chamber, thus disadvantageously imposing a relatively high load on a seal member fitted to the piston. The load imposed on the seal member by the power hydraulic pressure becomes a serious problem, particularly in the case where the brake hydraulic pressure receiving area and the power hydraulic pressure receiving area on the piston are made equal to each other in order to reduce the size of the apparatus and the power hydraulic pressure accumulated in the accumulator is set such as to be higher than a maximum value of the brake hydraulic pressure at which the wheel is brought into a state close to a locked state.